When a person sinks to the bottom of a pool of water, or even floats unconscious on the surface thereof, his lungs cease functioning and his brain is therefore deprived of oxygen. If this situation is allowed to continue for more than a few minutes, irreparable damage will be done to the brain even if resuscitation is effected. However, if the drowning person is detected quickly and extracted from the water, there is still a very short time period during which resuscitation is possible without permanent damage. Thus, rapid detection and identification of a drowning body is essential if resuscitation is to be at all possible and if permanent damage is to be avoided.
Clearly, detection of a drowning body which remains floating on the surface of the water may easily be effected manually by means of a life-guard, when present. Detection at the bottom of the swimming pool is rendered more difficult partly because it demands on the clarity of the water and also because when many people are swimming in a pool at the same time, it is difficult for a life-guard to monitor everybody's safety effectively.
Hitherto proposed methods and systems for determining swimmers in distress are generally based on detecting disturbances in the water and actuating a suitable alarm when such disturbances are detected. Such an approach is also used in order to monitor unauthorized entry into a swimming pool and to provide an alarm in the event of such unauthorized entry.
Thus, for example, U.S. Pat. No. 3,969,712 (Jerry et al.) discloses a device adapted to float in a swimming pool and sound an alarm when the water is disturbed and which includes a transducer for sensing noise in the water. The device is particularly sensitive to disturbances which are reflected off the walls of an average size swimming pool, other noises not being of interest and being filtered out.
Similarly, U.S. Pat. No. 3,953,843 (Codina) discloses a pool alarm system which includes a transmitter adapted to float on the surface of the water. When activated by a wave produced by a suitable disturbance, the transmitter emits a signal which is subsequently detected by a radio receiver which itself is coupled to an alarm which emits an audible signal.
Likewise, Australian Patent No. 8,821,727 (Webb) discloses a warning device for detecting the presence of a person in a pool of water and for detecting disturbances therein. When disturbances are detected and also a person is located within the swimming pool, an alarm is sounded. On the other hand, if disturbances are located but nobody is swimming in the pool, the alarm is disabled.
In contrast to the above patent, U.S. Pat. No. 3,732,556 (Caprillo et al.) discloses a swimming pool alarm system which provides an alarm as soon as a person or an object falls into the water. Caprillo et al. make no attempt to discriminate between a swimmer and an object and is intended primarily to protect against unauthorized tampering with the pool.
Thus, it will be apparent, that existing systems are intended to provide a warning in the event of unauthorized disturbances within the swimming pool. Clearly, such systems are effective only when the pool is empty. People swimming in the pool inevitably cause disturbances which require that alarm systems of the type described be disabled in order to prevent them from operating. Thus, alarm systems of this type are primarily intended to protect domestic swimming pools or even public swimming pools when they are not in use and the life-guard is absent.
The purpose of such alarm systems is generally to protect against unforeseen slipping into the water and to provide a warning that someone has entered the pool when it is, as yet, unauthorized for use. Specifically, systems of the type described exist which produce a warning signal when for example, small children slip into the water or when someone falls in as a result of darkness and is unable to climb out unaided.
Such systems are limited to detecting unforeseen disturbances within the pool and are not suitable for detecting the presence of somebody who is actually drowning. This is due, in part, to the fact that such systems must be disabled when the pool is authorized for use. If this is not done, then a drawback of prior art systems is that a false alarm will be produced. A further drawback is the incidence of false alarms as a result of non-human objects falling into the water and, indeed, the general inability of such systems to discriminate between human and non-human bodies. Even more significant is the inability of such systems to discriminate between swimmers, be they authorized or not, who have entered the pool and are experiencing difficulty from those who have entered the pool but are not in danger.
Observations of people drowning permit discrimination between such people and other bodies in the water. In particular, drowning bodies fall into two categories: most bodies which drown do so because, usually as a result of prior difficulties, the lungs absorb large quantities of water and the specific weight of the body becomes greater than that of water. In these cases, the body starts to sink and after a very short time period will settle unconscious at the bottom of the pool. Less commonly, it may happen that a swimmer loses consciousness before the lungs become full of water and, in these cases, the specific weight of the body remains less than that of the water and the body floats unconscious on the surface of the water. In either case, the violent disturbances which are caused by swimmers in distress are absent and, therefore, methods and systems which exploit such disturbances in order to detect a drowning person will not be effective.